High-speed data transmission over a twisted pair (i.e., subscriber local access line) such as HDSL, as it is presently carried out, uses transmission procedures full of complexity. Such complexities include multicarrier modulation, multitone transmission and orthogonal frequency division. These processes are all susceptible to external interference and to crosstalk.
Transmission of signal s(t) in a Twisted pair is subject to various types of interference including near end crosstalk (NEXT), which affects high speed data transmission. Multiple channels may be transmitted over a single twisted pair, but may interfere with one another. Twisted pair channels must be substantially orthogonal to one another in order to limit interference between channels.
A functional diagram of these interferences is shown in the FIG. 1A, where an input lead 101 represents application of the data signal s(t) to the twisted pair. The twisted pair may be characterized by a transfer function that is related to the absolute square of the functional value Hc(f), which represents an attenuation characteristic at the twisted pair, and is proportional to √{square root over (ƒ)}. In addition, there is significant interference from NEXT, represented by Ha(f). Far end crosstalk, FEXT, is relatively very low compared to NEXT and is not included in the model.
The input and the NEXT are applied to mixer 107 that represents the interaction of the signals. The output on lead 109 represents the amalgam of the data signal plus the interference signals.
The effect of this interference of the twisted wire attenuation of signals is shown by the graph of FIG. 1B where coordinate axis 100 of the log of signal frequency is plotted against a coordinate axis 102 representing attenuation in dB.
Curve 111 represents the attenuation of the data signal as function of frequency. Curve 113 represents the increase of NEXT as frequency increases. It is apparent that as the data signal attenuates with increasing frequency, and the interference signal increases with the increasing frequency. NEXT is a dominant portion of this interference. Other forms of interference include narrow band radio interference. All these interferences contribute to the frequency limits of the twisted pair. In a typical instance 95% of twisted wire capacity is below 10 kHZ and 60% of capacity is below 40 kHZ.